The present invention relates to an open head detector circuit for detecting an open-circuit condition in a magnetic head of a magnetic storage system.
In a magnetic storage system, such as a computer disk drive, digital information is magnetically stored upon the surface of a magnetic disk. The digital information is represented by selectively polarizing the magnetic field of consecutive areas across the surface of a rotating magnetic disk. When the information is read back from the storage disk, changes in the magnetic polarization of the medium are sensed and converted into data peaks in an electrical output signal. The reading and writing operations are performed through a magnetic read/write head which is suspended over the surface of the rotating disk.
The magnetic read/write head includes an inductive coil. Information is written on the disk surface by applying a strong current to the inductive coil. This current creates a magnetic field in either a first direction or in a second direction, depending upon the direction of current flow through the inductive coil. The magnetic field polarizes the magnetic field of the disk surface area directly adjacent the magnetic head. As the disk rotates, adjacent bit positions on the disk surface pass beneath the magnetic head and are polarized by the magnetic field induced by the magnetic head.
Digital information is recorded on the disk surface by reversing the polarization of selected bit positions on the disk surface. In one typical magnetic storage system, the digital information is encoded such that no more than 7 ZERO's are consecutively written to the disk surface. A digital ZERO in the encoded information is written by polarizing a particular bit position in the same direction as the previous, adjacent bit position. In contrast, a digital ONE is written by reversing the polarization the particular bit position so it is opposite to the polarization of the previous, adjacent bit position. A second digital ONE is written by again, reversing the polarization of the next adjacent bit position. The magnetic polarization is reversed by reversing the direction of current flow through the inductive coil in the magnetic head.
The digital information is read back from the disk surface by sensing changes, or reversals, in magnetic polarization from one bit position to the next. For example, no change in magnetic polarization represents a digital ZERO, while a reversal represents a digital ONE. As the magnetic disk rotates beneath the inductive coil, oppositely polarized bit positions passing beneath the magnetic head create a changing magnetic field across the inductive coil. This changing magnetic field induces current in the coil to create a data peak or pulse in an electrical output signal at the head contacts. The magnetic storage system monitors the electrical output signal and converts the signal to a digital representation, based upon the presence or absence of the data peaks.
Performance of the magnetic storage system depends greatly upon its ability to accurately and reliably read and write information onto the disk surface. When the magnetic head is not functioning properly, false data may be read from the disk or the information may not be written as intended.
One form of magnetic head malfunction is an open head condition. An open head condition occurs when the inductive coil breaks or does not make electrical contact with the head contacts. In other words, the inductive coil creates an open-circuit between the head contacts.
Magnetic storage systems typically include open head detection circuitry that can perform fault detection on the magnetic head. During operation, if the magnetic head malfunctions, the open head detection circuitry notifies the storage system of the malfunction by means of an open head fault signal to prevent the storage system from attempting to write data through the defective head. During operation, the storage system may use the open head fault signal to shut down the system and signal its operator to replace the defective head so that no data is lost. During manufacture, the detection circuitry may be used to test for faulty manufacturing connections and for head integrity before the storage system is shipped to the customer.
One method of detecting an open head condition includes threshold detection. Differential head voltages across the head contacts are measured and compared to a threshold voltage representative of a functioning head. During a write operation, a large current is forced through the inductive coil. In a functioning head, after write data circuitry reverses current direction, the differential head voltage initially becomes large. After a period of time, the differential head voltage drops until it stabilizes to a write current, head resistance product (I.sub.write * R.sub.head) before the next reversal in write current occurs. In a defective head, the differential head voltages never stabilize near the I.sub.write * R.sub.head product, but remain large.
The differential head voltage characteristics may be used to identify an open-circuit condition in the magnetic head by comparing the differential head voltage signals to the threshold voltage. If the differential signals are mostly higher than the threshold voltage, this indicates that the differential signals have not stabilized to the I.sub.write * R.sub.head product and that a fault condition exists.
Beck et al, U.S. Pat. No. 4,203,137, disclose an example of a threshold detector used in a magnetic read/write circuit. Beck et al disclose a magnetic head preamplifier with an electrical short detector for detecting electrical shorts in magnetic heads and head wiring. During write mode, the threshold detector monitors differential voltages across the magnetic head contacts. If the differential voltages are greater than a threshold voltage most of the time, the system is writing into a normal, operative head (not shorted). If the differential voltages are less than the threshold voltage for most of the time, this is an indication of a shorted head. The threshold detector disclosed in Beck et al may be adapted to detect open head conditions rather that head shorts.
Felleisen et al, in U.S. Pat. Nos. 4,754,222 and 4,656,420, disclose another example of the use of a threshold detector in magnetic read/write circuitry. Felleisen et al disclose a circuit arrangement for detecting faults on magnetic recording media. Recorded digital signals are sensed by a magnetic head and measured by threshold detectors to detect drop-ins (extra pulses) or drop-outs (missing pulses). The threshold detector determines whether data stored on the recording media has been lost or whether extraneous information has been stored on the media.
In another method of detecting an open head condition, a threshold detection circuit is used in combination with a timing circuit. In this method, the detector circuit determines whether the differential voltages exceed the threshold voltage for a fixed amount of time. Typically, the threshold voltage is selected to be slightly larger than the I.sub.write * R.sub.head product. The timing circuit is triggered by a write data signal that initiates a reversal in current flow through the head. The timing circuitry compares the differential head voltage to the threshold voltage at a given time after the initiation of the reversal in head current. If the differential head voltage has not yet stabilized to the I.sub.write * R.sub.head product, the open head detection circuitry flags a fault signal which halts disk drive operation. The disk drive notifies its host computer that the information was not stored as desired and maintenance is required.
Several problems arise with the open head detection circuits of the prior art. In practical applications, disk drives operate with wide ranges of head loads (head inductance), write currents and data rates. For example, the head inductance may vary from about 0.2 to about 3.0 micro-henries, depending upon the manufacturer. Physical parameters of the electronic components which form the write data circuitry may vary from one drive to the next, resulting in a variance in write current through the magnetic head. This causes a fluctuation between drives in the expected differential voltage ranges and the length of time the voltages take to stabilize. These fluctuations may be as great as an order of magnitude. Therefore, the threshold voltages and timing circuitry may not be accurate with respect to a particular drive.
As a result, open head detector circuits of the prior art are not fully reliable. These circuits may report false errors or fail to report valid errors. Further, these circuits are not easily adaptable to disk drives made by other manufacturers or in different product families. Each detector circuit must be specifically designed for the particular drive in which it is to be used.
There is a continuing need for improved open head detection schemes. It is desirable to have an open head detector circuit which is substantially insensitive to variations in physical parameters and to variations in data rates between the write data circuitry of different disk drives.